Article pubs.acs.org/JACS
Addition Reaction-Induced Cluster-to-Cluster Transformation: Controlled Self-Assembly of Luminescent Polynuclear Gold(I) μ3‑Sulfido Clusters Liao-Yuan Yao, Franky Ka-Wah Hau, and Vivian Wing-Wah Yam* Institute of Molecular Functional Materials and Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, P. R. China S Supporting Information *
ABSTRACT: Unprecedented addition reaction-induced gold(I) cluster-to-cluster transformation has been observed in the present work. Reaction of the chlorogold(I) precursor, [vdpp(AuCl)2] (vdpp = vinylidenebis(diphenylphosphine)) containing the diphosphine with unsaturated CC bond, with H2S resulted in a series of polynuclear gold(I) μ3-sulfido clusters bearing Au(I)···Au(I) interactions; the identities of which have been fully characterized by NMR, electrospray-ionization mass spectrometry, elemental analysis, and single crystal X-ray diffraction analysis. Diverse research methods, including UV−vis absorption, 1 H NMR, and 31P NMR spectroscopy, were employed to detect and monitor the transformation and assembly processes. Supported by single crystal structures, the existence of Au(I)···Au(I) bonding interactions sustains the diverse array of sophisticated polynuclear cluster structures and endues them with rich luminescence features.
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[vdpp(AuCl)2]10,11 (1), has been employed to construct luminescent polynuclear gold(I) μ3-sulfido clusters. The introduction of active sites is anticipated to bring about unexpected functions and properties to these clusters. Interestingly, triggered by addition reaction between the vinyl group and H2S, cluster-to-cluster transformation in this system has been observed, leading to the formation of four different types of clusters (Scheme 1). Similar activation of the CC bond of the vdpp ligand by metal complexation has been reported by Schmidbaur and co-workers.10 These newly constructed luminescent polynuclear clusters based on gold(I)−gold(I) interactions have been well characterized. The selfassembly and transformation of these clusters could be monitored via 31P NMR, 1H NMR, and UV−vis absorption spectroscopy. The present work demonstrates the possibility of constructing new gold(I) clusters on the basis of known cluster structures, which could largely expand the development of new libraries of novel gold(I) clusters and luminescent gold-based materials.
INTRODUCTION In the past few decades, there has been an increasing research interest in the self-assembly of polynuclear gold(I) complexes via gold(I)−gold(I) interactions.1−8 With a closed-shell electronic configuration of d10, gold(I) adopts a linear twocoordinate coordination geometry and tends to form strong inter- or intramolecular homonuclear interactions.1n,q,r This fascinating interaction, which has been termed aurophilic interaction by Schmidbaur,2c is found in a number of polynuclear aggregates that possess a wide diversity of configurations and rich photophysical properties. Due to the high stability and the relative accessibility of synthetic routes, chalcogenide-based polynuclear gold(I) complexes, especially sulfido gold(I) complexes, are among one of the most popular systems in the gold family.1f,6a−d,7,8 Since 1999, our group has developed a series of polynuclear gold(I) clusters of bridging phosphane ligands, with pyramidal [Au3(μ3-S)]+ units linked via metal−metal interaction directed self-assembly. Examples include the Au6,6a Au10,6b Au12,6c and Au18 clusters.6d In a number of these studies, there is the frequent coexistence of Au6 and Au10 clusters. Nevertheless, there was no evidence of the transformation between these gold(I) clusters. Although some examples of metal-based cluster transformation have been reported recently,9 to our knowledge, cluster-to-cluster transformations involving polynuclear gold(I) complexes are very rare. In this work, the chlorogold(I) precursor containing the unsaturated vinylidenebis(diphenylphosphine) (vdpp) ligand, © 2014 American Chemical Society
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RESULTS AND DISCUSSION Synthesis and Characterizations. Unlike previous studies in which reactions of H2S with phosphine chlorogold(I) precursors usually would give rise to clear solutions, reaction of 1 with H2S in this work initially led to a clear yellow solution Received: June 4, 2014 Published: July 17, 2014 10801
dx.doi.org/10.1021/ja505599v | J. Am. Chem. Soc. 2014, 136, 10801−10806
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Scheme 1. Proposed Self-Assembly of Gold(I) Clustersa
environments for the P atoms in solution. A hexagold(I) cluster, [Au6(μ-vdpp)3(μ3-S)2]Cl2 ([3]Cl2), was also isolated from the clear yellow solution. The yellow precipitate was demonstrated to be a pentagold(I) cluster, [Au5(μ-dppes)(μ3S)2]Cl ([5]Cl). Both of these two clusters were well characterized by 31P and 1H NMR spectroscopy, elemental analysis, and positive-ion ESI-MS. Structure Determinations. The solid state structures of clusters 2 and 4 were further confirmed by single crystal X-ray diffraction analysis. [2](PF6)2 crystallizes in the triclinic space group P1̅. Single crystal structure of [2](PF6)2 shows a decanuclear gold(I) μ3-sulfido cluster composed of four vdppAu2 units linked together by four S atoms, whereby opposite pairs of S atoms are bonded to the two gold atoms (Au(9) and Au(10)) at the core in a linear fashion (Figure 1). Alternatively,
Figure 1. Perspective view of the complex cation of [2](PF6)2, showing the atomic numbering scheme. Phenyl rings and hydrogen atoms have been omitted for clarity.
a The counter anions (Cl−) in the clusters have been omitted for clarity.
cluster cation 2 can be viewed as a Au8 macrocycle containing a Au2 chain at the center. The majority of Au···Au distances of [2](PF6)2 are in the range of 2.98−3.17 Å, suggesting significant intramolecular Au···Au interactions.1d−f The P−C− P angles of the vdpp ligands are about 113°, and in each ligand two phosphorus and two carbon atoms of the vinyl group are all placed approximately on one plane. Such planar geometry has also been observed in P−N−P ligands of previously reported clusters.6a,b Complex [4]Cl4 crystallizes also in the triclinic space group P1̅ and consists of two (dppes)Au4S2 units linked together by two (vdpp)Au2 connectors to give a dodecanuclear metallamacrocyclic cluster (Figure 2). Four gold atoms (Au(3), Au(4), Au(5), and Au(6)) form a distorted square in each (dppes)Au4S2 unit, which appears heart-shape from the side view. The Au···Au bond distances of [4]Cl4 are in the range of 3.00−3.21 Å, and relatively long Au···Au separations of 3.40 and 3.61 Å are found for Au(1′)−Au(6) and Au(2)−Au(4). As shown in Figure 2a, the top view of 4 is cross-shaped while the side view appears as two “hearts” connected by gold “chains” (Figure 2b). The bond lengths of the Au−P bonds in 2 and 4 are slightly shorter than 2.3 Å, while those of the Au−S bonds are slightly longer than 2.3 Å. In these two clusters, all the Au centers are two-coordinate with P−Au−S angles of 157.69(16) to 177.62(11)° that are distorted from the linear coordination geometry. The Au−S−Au angles range from 79.08(12) to 111.96(16)°, with most of them deviating from the 90°
with yellow precipitate observed upon continuous bubbling of H2S. The clear yellow solution afforded a yellow solid after evaporation. Vapor diffusion of diethyl ether into a dichloromethane-methanol solution of the yellow solid gave a mixture of yellow hexagonal block crystals and colorless needle-shaped crystals. 31P NMR and 1H NMR analysis of the yellow crystals indicated two different phosphorus and proton environments, with the molecular ion cluster observed at m/z 1841 in positive-ion ESI-mass spectrum, attributed to [Au10(μvdpp)4(μ3-S)4]2+ ([2]2+), which has been confirmed by X-ray crystallography. Similar to our previously reported Au10 cluster,6b complex 2 have a S4 point group symmetry. The two P environments could not be interchanged by any symmetry operations of the S4 point group, resulting in a pair of doublets in the 31P NMR spectra. 31P NMR analysis of the colorless needle-shaped crystals indicated the formation of a dodecagold(I) cluster, with a molecular ion peak at m/z 1234 ([4]4+) in the positive-ion ESI-mass spectrum. Both the 1H NMR and the ESI-MS data did not agree with the formation of [Au12(μ-vdpp)6(μ3-S)4]Cl4. The identity was eventually established by single crystal X-ray structure determination as [Au12(μ-vdpp)2(μ-dppes)2(μ3-S)4]Cl4 ([4]Cl4) (dppes = bis(2,2-bis(diphenylphosphino)ethyl)sulfane), in which vdpp was transformed to a new organic ligand via addition-reaction of the CC bond and H2S. The presence of a local D2h symmetry would account for the presence of two kinds of chemical 10802
dx.doi.org/10.1021/ja505599v | J. Am. Chem. Soc. 2014, 136, 10801−10806
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microsecond range of the radiative lifetime suggests the emission to be of a triplet parentage (Supporting Information, Table S3), which is tentatively assigned to a ligand-to-metal charge transfer (LMCT; S → Au) origin modified by AuI···AuI interactions or, alternatively, as a ligand-to-metal−metal charge transfer (LMMCT) excited state.6a−d Monitoring of the Transformation and Assembly Process. The existence of mixtures of clusters suggests the possibility of cluster-to-cluster transformation. Cluster reconfiguration might exist in the assembly process by the reaction with H2S. Attempts were made to preliminarily monitor the assembly process by UV−vis absorption spectroscopy. Complex 1 and cluster [5]Cl are only barely soluble in the reaction solution. In order to obtain a better understanding of the transformation and assembly, the progress of the reaction was monitored in DMSO solution. As the reaction proceeded in dichloromethane-ethanol-pyridine (3 mL, 1:1:1 v/v/v), aliquots of the reaction mixture (10 μL) were withdrawn, evaporated to dryness, and redissolved in DMSO for UV−vis absorption analysis. At about 5 min when the reaction mixture became a clear yellow solution, the absorption shoulder at around 300−350 nm rose dramatically from the baseline, corresponding to the formation of [2]Cl2 and [3]Cl2. When the reaction time reached about 15 min, two absorption maxima at ca. 310 and 360 nm, which could be unambiguously assigned to [4]Cl4, were observed. As large amounts of yellow solid started to precipitate (at about 20 min), the absorption maximum at about 360 nm decreased dramatically, with the absorption shoulder at around 310 nm remaining (Supporting Information, Figure S4), suggesting the transformation of the decanuclear gold(I) clusters to [5]Cl. To better uncover the mechanism of cluster-to-cluster transformation, NMR analysis including 1H and 31P NMR spectroscopy was employed to monitor the assembly process. Equal aliquots of the reaction mixture were withdrawn from the reaction mixture at different times. After evaporation to dryness, the solid residues were dissolved in deuterated DMSO for NMR studies. The proton signals of the vinyl or/ and the alkyl group (HA, HB, and HC in Scheme 1) were monitored. As shown in Figure 4a, signals of HA in 1 existed as a triplet at around δ 6.49 ppm. As the reaction proceeded, double−doublets at ca. δ 6.36 and 6.67 ppm attributed to protons of the vinyl group (HA) of [2]Cl2 and a triplet at δ 6.45 ppm assignable to HA of [3]Cl2 emerged. After about 5 min, the reaction mixture became clear and signals of 1 disappeared completely in the 1H NMR spectra. Then signals of [2]Cl2 and [3]Cl2 started to vanish gradually, and three new triplets of [4]Cl4 appeared at δ 2.30 (HC), 6.10 (HB), and 6.15 (HA) ppm. The upfield shift and the observed integral ratio of 4:2:2 demonstrated the generation of the new dppes ligand, indicating that two of the vdpp ligands have reacted with H2S to give rise to the coexistence of two types of ligands in [4]Cl4. Finally, only signals assignable to HB and HC of [5]Cl could be detected at δ 4.95 and 2.20 ppm after 30 min, which indicated the presence of only one type of phosphorus ligand, dppes, in the reaction mixture. Further supporting evidence came from the 31P NMR spectral changes (Figure 4b). The initial 31P NMR signal of 1 appeared at about δ 32.2 ppm. After gently bubbling H2S gas for around 3 min, the colorless suspension turned slightly yellow, and the 31P NMR spectra indicated the appearance of several new signals. The singlet at about δ 35.3 ppm could be assigned to [3]Cl2, while the pair of doublets at ca. δ 34.0 and
Figure 2. (a) Top and (b) side views of the crystal structure of the complex cation of [4]Cl4. Phenyl rings and hydrogen atoms have been omitted for clarity.
expected for bonding involving the sulfur 3p orbitals1f (Supporting Information, Tables S1 and S2). Photophysical Studies. The UV−vis absorption spectra of gold(I) clusters 2−5 show an intense absorption at around 266 nm (Supporting Information, Table S3), which could also be observed in the UV−vis absorption spectrum of the chlorogold(I) precursor and is assigned to a metal-perturbed intraligand transition, while the low-energy absorption shoulders at ca. 305−355 nm are tentatively assigned as ligand-to-metal chargetransfer (LMCT) transitions modified by Au(I)···Au(I) interactions. These clusters are emissive in solution state while the emission becomes more intense in the solid state under excitation wavelengths greater than 350 nm. At low temperature (77 K), the solids of these polynuclear gold(I) clusters mainly emit in the green-yellow region at ca. 506−565 nm (Figure 3; Supporting Information, Table S3). The
Figure 3. Low-temperature (77 K) solid-state emission spectra. 10803
dx.doi.org/10.1021/ja505599v | J. Am. Chem. Soc. 2014, 136, 10801−10806
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Figure 4. (a) 1H NMR and (b) 31P NMR spectral changes in DMSO-d6 during the assembly process.
monitoring experiments indicate that the most critical point of the transformation involves the addition reaction of the double bond of the vdpp ligand. In the current system, H2S would first react with the gold(I) precursor to form a mixture of hexa- and decanuclear gold(I) clusters. After the sulfide has coordinated to the gold(I) atoms, H2S would start to react with a portion of the vinyl groups, leading to the coexistence of two different phosphine ligands, vdpp and dppes. The increase in the rigidity and the steric hindrance of the organic phosphine ligands from vdpp to dppes would direct the cluster-to-cluster transformation from the decanuclear to the dodecanuclear gold(I) cluster, which is supported by the single crystal structures of cluster cation 2 and 4. According to the NMR analysis, the vinyl groups in cluster cation 4 would react with H2S and vdpp would be completely transformed to dppes in
29.5 ppm corresponded to signals of [2]Cl2. As the reaction mixture became clear at about 5 min, the 31P NMR spectra showed the completion of reaction between 1 and H2S. Then, at around 10 min, two singlets at ca. δ 42.2 and 33.2 ppm with integral ratio of 2:1 appeared, which corresponded to the formation of [4]Cl4. The signals of [2]Cl2 and [3]Cl2 subsequently disappeared, leaving only [4]Cl4 in the system. If H2S was bubbled continuously, large amounts of yellow precipitate appeared and only one singlet at around δ 42.6 ppm in the 31P NMR spectra was observed, corresponding to signals of the final product, [5]Cl. This addition reaction-induced cluster-to-cluster transformation is unprecedented and is the first of its kind. Similar reactions in the past with dppm and aminodiphosphine-bridged gold(I) systems did not yield similar phenomenon. The 10804
dx.doi.org/10.1021/ja505599v | J. Am. Chem. Soc. 2014, 136, 10801−10806
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H NMR (400 MHz, CDCl3, 298 K): δ 6.23 (dd, ABXY-system (A, B = H; X, Y = P), 4H, 3J(PH) = 30.7 Hz, 3J(PH) = 17.6 Hz, CH2), 6.36 (dd, ABXY-system (A, B = H; X, Y = P), 4H, 3J(PH) = 30.7 Hz, 3 J(PH) = 17.6 Hz, CH2), 6.80 (t, 8H, 3J(HH) = 7.7 Hz, −Ph), 7.08 (t, 8H, 3J(HH) = 7.7 Hz, −Ph), 7.19−7.25 (m, 8H, −Ph), 7.39−7.47 (m, 24H, −Ph), 7.53−7.62 (m, 20H, −Ph), 7.74 (t, 4H, 3J(HH) = 7.2 Hz, −Ph), 8.05 (dd, 8H, 3J(PH) = 13.1 Hz, 3J(HH) = 7.7 Hz, −Ph); 31 1 P{ H} NMR (162 MHz, CDCl3, 298 K): δ 29.84, 34.21 (dd, ABsystem (A, B = P), 2J(PP) = 146 Hz, vdpp), −144.22 (sept, PF6); positive ESI-MS: m/z: 1841 ([M]2+); elemental analysis calcd for [2](PF6)2 (found): C 31.44 (31.30), H 2.23 (2.32), N 0.00 (
